The present exemplary embodiment relates generally to electrical power conversion. It finds particular application in conjunction with motor drives, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
Power conversion systems convert electrical power from one form to another and may be employed in a variety of applications such as motor drives for powering an electric motor using power from an input source. Typically, the power converter is constructed using electrical switches actuated in a controlled fashion to selectively convert input power to output power of a desired form such as single or multi-phase AC of a controlled amplitude, frequency and phase to drive an AC motor according to a desired speed and/or torque profile, often in the presence of varying load conditions. In a typical situation, one or more AC motor drives are connected to an AC power bus or point of common coupling along with other loads where a common AC power source provides current to all these loads via the common bus. The AC drives may be equipped with power factor correction (PFC) apparatus to operate the drive at or near unity power factor. Such apparatus' generally include a number of DC bus capacitors that serve to store and release energy as needed by the load to maintain efficient operation.
The number of DC bus capacitors is at least in part determined by the voltage rating of the motor drive. For example, in one motor drive of a first rating a pair of capacitors can be placed in series to handle the given voltage of the drive. In another motor drive of a higher voltage rating, the same type of capacitors may be placed in three capacitor series with multiple parallel legs of capacitors to achieve the correct overall drive capacitance. This requires more capacitors. For example, a 480V motor drive may use 8 capacitors whereas a 690V motor drive may use 27 capacitors. Thus, as ratings increase, the number of capacitors needed to achieve the correct overall drive capacitance generally increases.
In a typical drive, the DC bus capacitors are mounted directly to the DC bus. As will be appreciated, as ratings increase a larger and larger space is consumed by the capacitors on the DC bus and within the motor drive enclosure. This has generally been addressed by increasing the size of the motor drive enclosure to accommodate the capacitors, as well as increasing the surface area of the DC bus to increase the space available to mount the capacitors.